The technical field of this invention is interleukin-2 receptors found on T-type lymphocytes (T cells) and, in particular, purified receptor proteins, processes for producing same, monoclonal antibodies having affinity to such proteins, and the use of such proteins and monoclonal antibodies for diagnostic and therapeutic purposes.
Interleukin-2 ("IL-2") is a 15,000 dalton glycoprotein lymphokine which is synthesized and secreted by some T cells following activation with an antigen or mitogen. Interleukin-2 is responsible for activated T cell growth cycle progression and initiates its effects by interacting with specific high-affinity membrane receptors. This interaction is critical to the normal immune response.
In 1982 a monoclonal antibody, termed anti-Tac, was reported by the present inventor and others which specifically bound to the human T cell membrane receptors for interleukin-2. See, Leonard et al., 300 Nature 267-269 (November, 1982), herein incorporated by reference, for a discussion of the techniques for preparing monoclonal anti-Tac. The anti-Tac antibody resulted from the immunization of mice with long term cultures of human T cells and was considered to be specific for the interleukin-2 receptor because of its ability to significantly block binding of radiolabeled IL-2 to the HUT-102 cells, a cell line derived from patients with acute T cell leukemia (ATL). The HUT-102 cell line is available from a wide variety of sources and has been used extensively by researchers.
At the same time, a receptor protein, present on normal T cells and a variety of leukemic cells, was identified and at least partially characterized by the present inventor and others. This receptor protein, containing the anti-Tac epitope, was isolated by immunoprecipitation using anti-Tac and found to have a molecular weight of about 55,000-60,000 daltons, as determined by gel electrophoresis. See the above-referenced Leonard et al. paper and U.S. Pat. No. 4,578,335 for further details on this receptor protein.
However, the same experiments that lead to the identification of the 55,000 dalton IL-2 receptor protein also revealed a very large discrepancy between the number of binding sites detectable using radiolabeled IL-2 versus radiolabeled anti-Tac. In particular, ATL cells lines, such as HUT-102, expressed 50-100-fold more anti-Tac binding sites than high-affinity IL-2 receptors. Subsequent experiments appeared to find a second class of sites that bound IL-2 with a 1,000-fold lower affinity (i.e., Kd=10 nM versus the high affinity sites with a Kd=10 pM). The great majority (greater than 98%) of the IL-2 binding sites found on the surfaces of T cells, especially ATL cell lines, were found to be of the low affinity category.
The confusion concerning the structure of interleukin-2 receptors was further compounded when ATL cell lines and the amino acid sequence of the anti-Tac purified receptor protein were used to identify cDNA clones that encoded the IL-2 receptor. After cDNA transfection experiments revealed solely low affinity IL-2 binding by transformed non T cells, but both high and low affinity IL-2 binding by T cells, it became apparent that an additional, T cell specific protein, was necessary to form high affinity IL-2 receptors. Thus far, investigators have searched for a component that would not of itself bind IL-2, but that would serve to convert the IL-2 binding of the 55,000 dalton Tac protein from low affinity to high affinity.
The structure of IL-2 receptors and their relationship to T cell growth and proliferation is of considerable scientific and clinical importance. T cells play a central role in the induction and regulation of the immune response. In particular, there exists a need to control T cell-induced immune responses in tissue and organ transplanted patients. Existing drug therapies, such as steroid hormones and cyclosporin, are at best indirect agents for immunosuppression. Agents that can block the interleukin-2 ligand-receptor interaction or alter receptor expression, would represent significant new weapons against tissue and organ graft rejection. Similarly, a wide range of autoimmune diseases, such as rheumatoid arthritis and other T cell-driven inflammatory diseases, could benefit from the development of a new class of T cell receptor antagonists. Moreover, a better understanding of the T cell receptor structure may also lead to improved treatments for immunodeficiencies such as those which occur in the acquired immunodeficiency syndrome (AIDS) and neoplastic T cell conditions.